The present invention pertains to a galvanic cell having a negative zinc electrode, an alkaline electrolyte, and a housing which is sealed by a sealing member made of plastic and having a predetermined breaking point in the form of a bursting membrane or a score.
The present invention generally relates to cells of the alkaline-manganese type having a housing in the form of a round cell. The housing of this cell is preferably formed as a steel cup for receiving the electrode materials, and is sealed in liquid-proof manner by a plastic seal. The steel cup acts as the positive terminal and cell pole.
To protect against explosion of such a cell, which may occur as the result of unacceptably high internal pressures due to overheating of the cell, or due to incorrect connection of the cell poles, the cell's plastic seal is advantageously provided with predetermined breaking points. To this end, points of reduced strength are provided which can withstand normal operating pressures, but which can assure the release of excess pressures from the cell by breaking down before the cell's internal pressure reaches an uncontrollably high value.
Predetermined breaking points in plastic seals are usually formed by greatly reducing the thickness of the material in a desired area. As an example of this, the sealing member described in U.S. Pat. No. 4,476,200 includes a bursting membrane to develop the desired predetermined breaking point. However, the predetermined breaking point may also be developed as part of the cell's disk-shaped sealing member, by providing the sealing member with a region having a smaller wall thickness than its surroundings and which is separated from its surroundings by a circular score or rupture line of reduced material thickness.
Care must be taken to assure that the integrity of the plastic seal is not jeopardized by the predetermined breaking point. Its chemical resistance to attack by the electrolyte must be ensured. Its stability in terms of shape, which is needed to maintain internal pressure, must not be allowed to decrease. The rate of permeation for oxygen, water and electrolyte must be maintained low.
Insofar as it was possible to take these fundamental requirements into account, of the available materials, polyamide proved to be especially suitable for use with zinc-manganese dioxide cells as compared to other thermoplastics such as polyethylene, polypropylene, polyethylene-polypropylene copolymers, polyphenylene oxide or acrylonitrile-butadiene-styrene terpolymer because polyamide is characterized by high dimensional stability and satisfactory resistance to chemicals. Because of these properties, seals based on polyamide have also proven to be especially highly successful in connection with the manufacture of round cells.
However, practical experience has shown that the bursting pressure drops to only a fraction (about 1/10) of its original value in the case of polyamide seals provided with bursting membranes if the seal is simultaneously exposed to alcaline electrolyte and elevated temperature. This reduction, which leads to a premature response of the bursting membrane, was found to result from stress corrosion cracking which develops under the effect of the electrolyte.